It is well known that methyltetrahydrophthalic anhydride can be produced by Diels-Alder reaction between maleic anhydride and a C.sub.5 fraction.
For example, 3-methyltetrahydrophthalic anhydride (hereinafter referred to briefly as 3Me--THPA) is obtained when trans-1,3-pentadiene is used as the C.sub.5 fraction and 4-methyltetrahydrophthalic anhydride (hereinafter referred to briefly as 4Me--THPA) is obtained when isoprene is used as said C.sub.5 fraction.
The C.sub.5 fraction to be used for the production of such methyltetrahydrophthalic anhydride need not be in a purified form and is commonly used in the form of a crude fraction as the diene component. This is due to the fact that, while the trans-1,3-pentadiene, for example, separated from the C.sub.5 fraction produced by naphtha cracking and the like usually contains olefins, diolefins and paraffins such as cis-1,3-pentadiene, pentanes and pentenes as impurities, trans-1,3-pentadiene substantially selectively participates in the Diels-Alder reaction.
While the use of such a crude fraction is thus economically advantageous, it entails the disadvantage that mainly cis-1,3-pentadiene and other olefins (and also a portion of trans-1,3-pentadiene or isoprene) tend to copolymerize with maleic anhydride to form a gel-like byproduct.
The hitherto-proposed methods for overcoming such a problem include the addition of a radical polymerization inhibitor such as hydroquinone (Japanese Examined Patent Publication No. 5537/1970), the use of benzene, toluene, xylene or the like as a diluent (Japanese Unexamined Patent Publication No. 92932/1979) and the concomitant use of a different kind of cyclic acid anhydride (Japanese Unexamined Patent Publication No. 39082/1981).
However, even the use of a radical polymerization inhibitor does not necessarily result in a complete prevention of the formation of a gel-like byproduct and, moreover, discolors the product. The use of a diluent which is poor in reactivity necessitates a step for removing and recovering the diluent after completion of the reaction, and entails an economic disadvantage. When a different cyclic acid anhydride is used in combination, the unit production is decreased and, therefore, the method is not advantageous economically.